The Definition of “graph” and Terminologies
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graph is a nonlinear data structure consisting of vertices and edges. There are a lot of terminologies to describe a graph. If you encounter an unfamiliar term in the following Explore Card, you may look up the definition below.
 Vertex: In Figure 1, nodes such as A, B, and C are called vertices of the graph.
 Edge: The connection between two vertices are the edges of the graph. In Figure 1, the connection between person A and B is an edge of the graph.

Path: the sequence of vertices to go through from one vertex to another. In Figure 1, a path from A to C is [A, B, C], or [A, G, B, C], or [A, E, F, D, B, C].
Note: there can be multiple paths between two vertices.
 Path Length: the number of edges in a path. In Figure 1, the path lengths from person A to C are 2, 3, and 5, respectively.
 Cycle: a path where the starting point and endpoint are the same vertex. In Figure 1, [A, B, D, F, E] forms a cycle. Similarly, [A, G, B] forms another cycle.
 Negative Weight Cycle: In a “weighted graph”, if the sum of the weights of all edges of a cycle is a negative value, it is a negative weight cycle. In Figure 4, the sum of weights is 3.
 Connectivity: if there exists at least one path between two vertices, these two vertices are connected. In Figure 1, A and C are connected because there is at least one path connecting them.
 Degree of a Vertex: the term “degree” applies to unweighted graphs. The degree of a vertex is the number of edges connecting the vertex. In Figure 1, the degree of vertex A is 3 because three edges are connecting it.
 InDegree: “indegree” is a concept in directed graphs. If the indegree of a vertex is d, there are d directional edges incident to the vertex. In Figure 2, A’s indegree is 1, i.e., the edge from F to A.
 OutDegree: “outdegree” is a concept in directed graphs. If the outdegree of a vertex is d, there are d edges incident from the vertex. In Figure 2, A’s outdegree is 3, i,e, the edges A to B, A to C, and A to G.
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